http://xuesql.cn/
https://sqlbolt.com/lesson/select_queries_with_joins
https://www.w3resource.com/sql-exercises/sql-retrieve-from-table.php#SQLEDITOR
http://xuesql.cn/static/%E9%87%91%E8%80%81%E5%B8%88%E6%89%8B%E5%86%8C.html
将要做：
https://www.w3resource.com/sql-exercises/sql-retrieve-from-table.php#SQLEDITOR
一些问题记录：
https://sqlbolt.com/lesson/select_queries_with_nulls
image.png
https://www.w3resource.com/sql-exercises/sql-retrieve-from-table.php#SQLEDITOR [28]
